1. Field of the Invention
The present invention relates to a pressure sensor, and more particularly, to a pressure sensor for electronic skin capable of measuring the amount of pressure applied thereto using pressure-sensitive conductivity, and a manufacturing method thereof.
2. Description of the Related Art
Artificial or electronic skin that uses pressure-sensitive or pressure-activated conductive rubber is being researched and developed. When a predetermined amount of pressure is applied to human skin, not only can the skin sense the application of pressure, but it can also sense the intensity of the pressure. It is important that electronic skin using pressure-activated conductive rubber performs the same functions as human skin.
Pressure-activated conductive rubber, which is used in pressure sensors, gains conductivity when pressure is applied to it and loses conductivity when the pressure is removed.
However, while pressure-activated conductive rubber is apt for use in an on/off type sensor (due to its abrupt build-up and loss of conductivity), it is unsuitable for pressure intensity detection. Accordingly, electronic skin that is manufactured using pressure-activated conductive rubber can sense whether there is pressure applied, but unlike real skin, cannot determine the amount of pressure applied.
FIG. 1 is a schematic view of a conventional pressure sensor using pressure-activated conductive rubber.
Referring to FIG. 1, the conventional pressure sensor includes pressure-activated conductive rubber 1, electrodes 2 and 3, a voltmeter 10, and an ammeter 20.
The pressure-activated conductive rubber 1 is a bendable material that has conductive characteristics by suddenly increasing conductivity when pressure exceeding a predetermined amount is applied thereto.
The electrodes 2 and 3 are respectively formed on either side surface of the pressure-activated conductive rubber 1. After a voltage is applied through the voltmeter 10, whether pressure applies on either side surface of the pressure-activated conductive rubber 1 can be detected by measuring current flowing through the ammeter 20. Specifically, when there is no pressure applied to any area of the surface of either side of the pressure-activated conductive rubber 1, the pressure-activated conductive rubber 1 exhibits insulating characteristics so that current flows through the ammeter 20. Conversely, when a pressure exceeding a predetermined amount is applied to at least one of the two side surfaces of the pressure-activated conductive rubber 1, the pressure-activated conductive rubber 1 displays conductive characteristics, whereby current flows through the ammeter 20. Therefore, the pressure sensor configuration illustrated in FIG. 1 can detect whether pressure applies to either end of the electrodes 2 and 3.
FIG. 2 is a graph illustrating the output characteristics of the pressure sensor of FIG. 1. As shown in FIG. 2, because the pressure-activated conductive rubber 1 decreases its resistance abruptly when a predetermined pressure is applied thereto, the range in which the pressure can be measured is extremely narrow. Accordingly, conventional pressure sensors can only determine if there is an applied pressure, and cannot emulate the ability of human skin to sense the amount of applied pressure. For example, when pressure-activated conductive rubber is installed on robotic fingers for lifting an object (an egg, for example), the object is placed between the fingers, after which the gaps between the object and the fingers are narrowed until the pressure-activated conductive rubber is pressed. Here, if x is the distance that the pressure-activated conductive rubber is depressed and the elastic constant is k, then the following equation 1 may be derived.
                    PRESSURE        =                  Kx                      contacted            ⁢                                                  ⁢            surface            ⁢                                                  ⁢            area                                              Equation        ⁢                                  ⁢        1            
When the pressure applied to the pressure-activated conductive rubber 1 (calculated using Equation 1) is greater than a pressure at which an egg slips and less than the pressure at which the egg cracks, the closing movement of the two robotic fingers is stopped and the egg may be held.
However, if another object that is heavier than an egg (for example, a billiard ball) is to be held after the egg, an alternate pressure sensor must be installed.